Method of sintering



Aug. 2, 1966 D. M. SCRUGGS 3,264,102

METHOD OF SINTERING Filed March 25, 1962 3 Sheets-Sheet l 2000 2400 28 0 3200 560a qooo 4400 4800 5200 5600 6000 6400 TFMPEPATU?! "F 1N VEN TOR.

ATraPA/E) Aug. 2, 1966 D. M. SCRUGGS METHOD OF SINTERING- Filed March 23, 1962 2 Sheets-Sheet 2 S N k 6 5 States Unite The present invention relates to an improved method of sintering, as well as to the materials that are made by the method.

Applicant has discovered that metals that are sintered at very high vacuums (i.e., a vacuum whose absolute pressure is below the vapor pressure of the metal at the sintering temperature) have certain properties that are considerably better than the properties of the same metal sinter by processes that have been used heretofore. The metals sintered according to the principles of the present invention have better ductility, mal-leability, better thermal shock resistance, and exhibit a desirable combination of good strength and modulus of elasticityin the as sintered, porous state. Materials sintered according to the principles of the present invention have the further advantages, over materials sintered according to prior art methods, in that they develop high bond strengths with short sintering times, and have higher purity. Applicant has further discovered that even though his improved method of sintering involves working under conditions which produce a deterioration of the material being sintered, namely evaporation, the sintering conditions produce such strong sintered bonds, rapidly enough so that the amount of catastrophic deterioration which is experienced will normally be an acceptable low value. According to further principles of the present invention, applicant has found that this relatively low value of catastrophie deterioration can be further reduced by certain procedures, later to be described, among which is the addition of a low vapor pressure material which does not appreciably react with the pure metal, but which will react with other impurities in the metal to form a protective coating during sintering. In some instances this coating may be superficially applied as will later be described.

An object of the present invention is the provision of a new and improved method of sintering metals which performs a unique sintering operation wherein the materials sintered have better physical properties than the materials sintered by the processes used heretofore.

A further object of the present invention is the provision of new and improved methods of reducing the amount of catastrophic deterioration that is experienced during applicants improved sintering process.

A still further object of the present invention is the provision of a new and improved method of sintering wherein sinter bonds are formed under conditions wherein the vapor pressure of the metal is above the environmental pressure.

Still further an object of the present invention is the provision of simple and novel means for providing a protective coating around the materials to be sintered.

Further objects and advantages of the invention will become apparent to those skilled in the art to which the invention relates upon reading the description of the principles that are involved, and which are described partially with reference to the accompanying drawings forming a part of this specification, and in which:

FIGURE 1 is a semi-log plot of the vapor pressure vs. temperature of various representative metals;

FIGURE 2 is a photomicrograph showing the structure of a composite metal ceramic material that was prepared according to principles of the present invention at a magnification of 300 diameters; and

FIGURE 3 is a cross section through a die cavity showing a filling tube installed therein; and by reason of which, a dissimilar powder material may be positioned around the metal which is to be sintered, and which is placed within the tube. After filling, the tube is removed and the powder is compressed.

As has been previously stated, an improvement in the physical properties of sintered metals is obtained by sintering the metal in an absolute pressure which is less than the vapor pressure of the metal being sintered. The following example demonstrates some of the improvement which is obtained by sintering under these conditions:

EXAMPLE I A 325 mesh electrolytic chrome powder was compacted at 20,000 p.s.i. and then sintered for 2 hours at 2900 F. in a 1,000 micron vacuum. The material was sintered in a 10 inch General Electric, Model 40D, tungsten element, cold wall furnace which was specially constructed, and which was evacuated by a 4-stage pumping system. The pumping system comprises a 10 inch Stokes diifusion pumpModel 160; a 6 inch Stokes booster pump-Model 150; a Welsh duo seal holding pump-Model 14200-B; and a Kinney rough'ening pumpModel KC-46. This equipment has the ability not only of producing vacuums down to the ranges of .1 to .01 of a micron, but in addition has the capacity for evacuating what would normally be considered large quantities of vapors. Approximately 35% of the original material was lost during the sintering process, and that which remained had a density of 5.2 grams per cubic centimeter to give a porosity of 38%, and it exhibited a malleability of approximately a 50% reduction in length when cold pressed before cracking.

In another instance, pure chromium sintered at 2900 F. in a vacuum of 1 micron absolute pressure had a 50% weight loss, a porosity of 39% and likewise gave a 50% reduction when pressed at room temperature before cracking.

By way of contrast, prior art materials that have been sintered in hydrogen, or argon gas, were not malleable at room temperatures, and these materials only had a transition to a ductile nature when heated to temperatures above 500 F.

Other materials can be added to the chromium without detracting from the advantages which are derived by vapor phase sintering; and these materials may be added to perform one or more purposes, such as: alloying to increase strength, or to improve oxidation resistance. One such material is titanium.

EXAMPLE II The following mixture of powders was compacted at 20,000 p.s.i. and then sintered for 2 hours at 2900 F. in a vacuum of 1 micron absolute pressure. Approximately 34% of the material was lost during sintering guard that which remained had a porosity of approximately 99.5% of 325 mesh electrolytic chrome .5% of 325 titanium After sintering, the above material was heated up to a temperature of 2200 F. in an argon atmosphere and was consolidated by extrusion process to give a reduction in cross section of After extrusion this material exhibited a capability of 10% elongation at room temperature.

Chromium has an appreciable vapor pressure at temperatures below its melting point so that a good percentage of the material may be lost when subjected to high temperatures in a vacuumas demonstrated by the Examples I and II above. has in general not believed it feasible to process chromium in a vacuum. In one reported instance, a prior art worker tried pouring molten chromium in a vacuum, with the result that practically all of the chromium was lost during the pouring process. Applicant probably would not have pursued his experiments had he not first tried to sinter a chrome-ceramic composite in a vacuum during which there was only approximately a 12% weight loss.

EXAMPLE III The following mixture of materials was compacted at 20,000 p.s.i. and sintered for 2 hours at 2900 F. in a vacuum of 1 micron absolute pressure:

93.5% of a 325 mesh electrolytic chromium powder 6% of 325 mesh magnesium oxide .5 of 325 mesh titanium powder After sintering, the material had a porosity of approximately 11%, showed a malleability at room temperature of 70% and had experienced approximately 12% weight loss during sintering. FIGURE 2 of the drawing is a photomicrograph of such a material. In FIGURE 2, the black areas are voids, and the grey areas 12 are ceramic. The voids and ceramic are located in the grain boundaries, and the voids comprise approximately 11% by volume in the as sintered condition before hot pressing.

Work with chrome composite materials has shown that the amount of weight loss is generally a function of the surface area that is exposed to the vacuum; and further shows that only a few percent of the material is removed from the internal pores of the material during the sintering operation. Work with metal-ceramic composites has shown that a thin layer of porous oxides forms on its surface to reduce the amount of surface that is exposed to the vacuum. The layer of oxides are porous enough to permit the vacuum to be transmitted through its pores to the internal pores of the material being sintered, and thereby allows the internal pressure of the material to be reduced to the environmental vacuum. Practical means therefore of cutting down the weight loss which occurs during vacuum sintering is had by the low vapor pressure oxide complex that is formed around the surface of the material during sintering.

While the protective oxide complex coating was formed in the above example by the inclusion of a metal oxide throughout the body being sintered, any means of forming a porous adherent low vapor pressure coating around the body to be sintered can be used to reduce the weight loss during sintering. One such means is to provide a flame sprayed coating of a ceramic around the material. Suitable examples of such coating are zirconia, alumina, thoria, or a mixture of MgO and SiO Any suitable ceramic materials can be used with any metal, and the following are given by way of example only:

Where chromium is to be protected, the flame sprayed coating may consist of a mixture of magnesium oxide and silicon dioxide, or zirconia;

Where manganese is to be protected, zirconia, or alumina;

Where beryllium is to be protected, zirconia can be used.

All that is required is that: the ceramic must have a lower vapor pressure than the metal and not melt during the sintering process; the ceramic must remain porous during the sintering process; and the ceramic should not react to any great extent with the metal being protected.

Another method of providing protection during sintering is to provide a metal alloying ingredient which will develop a more passive oxide surface during sintering. By this means low vapor pressure oxides can be provided For this reason the prior art for metals which normally sublime. For those metals which produce scale having poor adherence, alloying ingredients can be added which are known to improve the stability of the scale.

In some instances the protective coating can be formed by spraying or otherwise applying an aqueous solution of a salt such as magnesium chloride, titanium hydride etc., which will later decompose to provide the alloying ingredient. This same technique can be used in some instances to apply a coating of oxides which will quickly change to a porous adhering layer during the sintering process.

In still other instances the porous protective surface can be provided by the process demonstrated in FIGURE 3 of the drawings. In the process shown in FIGURE 3, a thin walled tube having a diameter slightly less than that of the die cavity in which the materials are to be compacted, is inserted into the die cavity. The tube may be suitably formed from shim stock for example. A suitable metal or ceramic power that is to be used for the coating may be poured across the bottom of the die cavity as well as into the annular space between the tube and the sidewalls of the die cavity. Thereafter the metal powder to be sintered, as for example, chromium, beryllium, titanium, columbium or any other suitable metal is poured into the inside of the hollow tube-following which, the tube is withdrawn. A top layer of the protective material may be poured across the top surface of the material to be sintered, either before, or after, the tube is withdrawn. Thereafter the material is compressed, fabricated, and sintered in the manner described above.

Sintering according to the principles of the present invention not only can be used to provide metals of increased purity, ductility, and strength, but can be used to provide sintered articles of high porosity that will possess malleability in spite of porosity. On the other hand when it is desired to eliminate the porosity, materials of high theoretical density can be provided by the step after sintering, of hot pressing to close the voids. If the hot pressing is done at about the sintering temperature, and the material is allowed to remain at this temperature for a short period of time following the pressing operation, the voids will be welded shut and a high density achieved. As indicated above any metal can be sintered according to the principles of the present invention at a temperature below its melting point and at an absolute pressure below the vapor pressure of the metal being sintered. FIGURE 1 of the drawings shows the vapor pressure temperature curves for a good number of metals which can be sintered according to the principles of the present invention; and it is an easy matter to select the correct sintering conditions from the graph provided in FIGURE 1.

EXAMPLE IV A 325 mesh electrolytic beryllium powder was sintered for 1 hour at 2300 F. in a vacuum having an absolute pressure of 0.1 of a micron. This material had a porosity of 28% and gave a malleability under slow hydraulic pressing at room temperature of 25%.

EXAMPLE V The following mixture of metals was compressed at 20,000 p.s.i. and sintered for 1 hour at 2300 F. in a vacuum of 0.1 micron absolute pressure:

99% of 200 mesh electrolytic beryllium 1% of -325 mesh aluminum fit that is derived by sintering in an absolute pressure below the vapor pressure of the metal being sintered.

Where it is desired to produce an alloying of the materials during the sintering process, a particle size of to 40 micron (325 mesh) will assure practically complete alloying in the short sintering times that are used in applicants improved sintering process. Where it is desired to produce porous articles, an increase in porosity is obtained by increasing the particle size. In general, the amount of precompaction that is used is not critical; inasmuch as it is very difiicult, if not impossible, to compress the particles to a degree wherein insufficient porosity exists to subject the internal structure of the material being sintered to the environmental vacuum.

The degree to which the absolute pressure should be less than the vapor pressure of the material being sintered to some extent affects the properties of the sintered article. The absolute vapor pressure of the metal may be approximately equal; but in general, an absolute pressure of from 0.1 to 0.001 of the vapor pressure of the material being sintered is to be preferred in order to achieve better sintered properties.

It will be apparent that the objects heretofore enumerated as well as others have been accomplished; and that there has been provided a new and improved method of sintering which produces materials having better physical properties than those produced by prior art sintering methods.

While the invention has been described in considerable detail, I do not wish to be limited to the particular examples above described; and it is my intention to cover hereby all novel adaptations, modifications, and arrangements of the principles disclosed which will occur to those skilled in the art.

I claim:

1. An improved method of sintering finely divided metals, said method comprising: determining the vapor pressure of the metal at the sintering temperature, selecting a metal oxide that does not melt at the sintering temperature of the metal; forming a porous coating on the metal to be sintered with said metal oxide, and sintering the metal in a vacuum Whose absolute pressure is below the vapor pressure of the metal.

2. An improved method of sintering mixtures of finely divided metals and ceramics, said method comprising: determining the vapor pressure of the metal constituent at the sintering temperature, selecting the ceramic constituent of lower vapor pressure than the metal constituent and having a melting temperature greater than the sintering temperature, and sintering the mixture in a vacuum whose absolute pressure is below the vapor pressure of the metal.

3. The method of sintering metal particles comprising: selecting a sintering temperature for the metal particles at which temperature the metal particles has a predetermined vapor pressure, selecting a ceramic which has a vapor pressure considerably below said predetermined vapor pressure at said sintering temperature and which Will form a porous coating, coating the metal particles to be sintered with said selected ceramic, and sintering said coated metal particles in a vacuum whose absolute pressure is below said predetermined vapor pressure.

4. An improved method of vacuum sintering chromium metal particles, said method comprising the steps of: selecting a powdered ceramic having a vapor pressure lower than said chromium metal particles, forming a protective porous coating on said chromium metal particles with said powdered ceramic, sintering the coated chromium metal particles at an absolute pressure less than the vapor pressure of the chromium metal particles at the sintering temperature.

5. An improved method of vacuum sintering chromium particles, said method comprising the steps of: selecting a powdered ceramic from the group consisting of magnesium oxide, silicon dioxide, zirconia or mixtures thereof, forming a protective porous coating on said chromium particles with said powdered ceramic, sintering the coated chromium particles at an absolute pressure less than about one-tenth of the vapor pressure of chromium particles at the sintering temperature.

6. An improved method of vacuum sintering beryllium particles, said method comprising the steps of: selecting a powdered ceramic having a vapor pressure lower than said beryllium particles, forming a protective porous coating on said beryllium particles with said powdered ceramic, sintering said coated beryllium at an absolute pressure less than the vapor pressure of beryllium particles at the sintering temperature.

'7'. An improved method of vacuum sintering manganese metal particles, said method comprising the steps of: selecting a powdered ceramic having a vapor pressure lower than said manganese metal particles, forming a protective porous coating on said manganese metal particles with said powdered ceramic, sintering the coated manganese metal particles at an absolute pressure less than the vapor pressure of manganese metal particles at the sintering temperature.

8. An improved method of vacuum sintering chromium metal particles, said method comprising the steps of: selecting a powdered ceramic having a vapor pressure lower than said chromium metal particles and a melting point greater than 2900 F., forming a protective porous coating on said chromium metal particles with said powdered ceramic, sintering said coated chromium metal particles at a temperature above approximately 2900 F. in a vacuum whose absolute pressure is less than approximately 1000 micron of Hg pressure.

References Cited by the Examiner OTHER REFERENCES Article by Cox, Vacuum sintering, contained in Metal Industry, Sept. 2, 1960, pp. 186-189.

Goetzel, Treatise on Powder Metallurgy, vol. I, In-

terscience Publishers Co., Inc., New York, 1950, pages 218 and 224.

LEON D. ROSDOL, Primary Examiner.

REUBEN EPSTEIN, CARL D. QUARFORTH,

Examiners.

R. L. GOLDBERG, R. L. GRUDZIECKI,

Assistant Examiners. 

1. AN IMPROVED METHOD OF SINTERING FINELY DIVIDED METALS, SAID METHOD COMPRISING: DETERMINING THE VAPOR PRESSURE OF THE METAL AT THE SINTERING TEMPERATURE, SELECTING A METAL OXIDE THAT DOES NOT MELT AT THE SINTERING TEMPERATURE OF THE METAL; FORMING A POROUS COATING ON THE METAL TO BE SINTERED WITH SAID METAL OXIDE, AND SINTERING THE METAL IN A VACUUM WHOSE ABSOLUTE PRESSURE IS BELOW THE VAPOR PRESSURE OF THE METAL. 